Methods for matching colors on a first device, such as a monitor that uses the RGB color space to render colors, and a second device, such as a printer that uses the CMYK color space to render colors, are well known in the art. In the method, for example, first a color conversion table describing correlations between parameters in the RGB color space (RGB values) and parameters in the CMYK color space (CMYK values) is prepared, then convert data in the RGB color space to data in the CMYK color space by using this color conversion table.
Specifically, a personal computer (hereinafter abbreviated as “PC”) issues a command to a printer to print a test patch image on recording papers. The test patch image includes a plurality of test patches, each of a different color. A patch number is assigned to each test patch. The CMYK test data used for outputting the test patches is also stored on the PC for determining correlations between each test patch and CMYK values used for outputting the test patch. Next, the PC measures colorimetric values of test patches in the test patch image and creates data indicating correlations between RGB values and CMYK values based on the measured values for each test patch and the CMYK values of the test data.
The PC processes a conical space as shown in FIG. 8(a). In FIG. 8(a), red (R), magenta (M), blue (B), cyan (C), green (G), and yellow (Y) have been set as the primary colors in the conical space. First, the PC performs a process on the hue of one primary color extracted from the conical space as shown in FIG. 8(b) (the color blue in the example shown in FIG. 8(b)), and uses the measured colorimetric values from the test patches to find correlations between RGB values and CMYK values at various points along an achromatic axis 100 between points P3 (white) and P0 (black), a side 102 between points P0 and P7 (blue), and a side 104 between points P7 and P3. After finding RGB values and CMYK values at points along these lines, the PC next performs linear interpolation to calculate correlation data between RGB and CMYK values for points in the color gamut defined by the achromatic axis 100 and sides 102 and 104.
As shown in FIG. 8(b), the PC finds a cyan value Cx for a point X in the color gamut according to the following Equations (1)-(3) using cyan values corresponding to points R, S, P, and Q for which correlations are known.CX1=CP+(CQ−CP)*a/b  (1)CX2=CR+(CS−CR)*c/d  (2)CX=(CX1+CX2)/2  (3)
Here, “CP” is the cyan value for point P, “CQ” the cyan value for point Q, “CR” the cyan value for point R, and “CS” the cyan value for point S. Further, “a” is the length of the line segment PX, “b” the length of the line segment PQ, “c” the length of the line segment RX, and “d” the length of the line segment RS.
Values for each primary color magenta, yellow, and black can similarly be found for a grid point X. By calculating values corresponding to each of the CMYK colors for grid points in the color gamut, correlations between RGB and CMYK values can be found for the entire selected color gamut (blue in this example).